Thursday, October 30, 2014

I'm watching the potential for what could be a significant winter storm around the November 5-8 period.

PSU

The image above shows a four-panel forecast from the GFS model, valid on November 7th. The top-left panel of this image shows 500mb vorticity values, where reds show positive vorticity (trough/low pressure) and purples depict negative vorticity (ridging/high pressure). On the top-right panel, we see the forecasted mean sea level pressure contours, with appropriated high and low pressure demarcations. The red dashed line in the US shows, in essence, the rain/snow line. Since we are in the winter season, temperature profiles might not be as supportive of snow as one may think, but we'll address that later. The bottom-left panel shows relative humidity values in the lower levels of the atmosphere, while precipitation is seen in the bottom-right panel.

What do we notice about this forecast? There's a massive low pressure system barreling into the Northeast, with enough cold air in its wake to indicate the threat for heavy snow, according to this GFS forecast. This strong storm system looks to originate from Canada before gathering its strength along the coast, as many coastal storm systems do.

According to this forecast, the storm would certainly drop big snows, but let's see if other model guidance is catching on.

PSU

As a matter of fact, other guidance does support this idea. The above image shows the ECMWF's depiction of the atmosphere, valid for the same timeframe as the GFS image, and the first thing we see is one heck of a storm. In the 500mb height anomaly panel on the top-left, where blues depict ridging/high pressure and reds show troughs/low pressure, it's quite apparent that a major storm is overtaking the Northeast. The MSLP chart on the top-left once again shows a very strong storm system, similar to the GFS projection. Strong lower-level winds in the bottom-left panel, and sub-freezing temperatures only a few thousand feet above the ground confirm that this would likely be a very strong winter storm, IF this forecast were to verify, in line with the GFS.

Are there any other model guidance systems showing this sort of winter storm?

Yes.

PSU

The chart above shows the 180-hour projection from the NAVGEM model, with each panel the same parameters as those on the GFS chart earlier in this post. This model only goes out to 180 hours on this particular web site, but when we compare the 180-hour forecasts from the GFS and ECMWF models, the atmospheric set-up prior to this storm at 240-hours is incredibly similar among the three model guidance.

Those of you who are weather enthusiasts know that the NAVGEM (formerly the NOGAPS) model isn't worth putting much stock in; I'm mentioning it here just for pure discussion.

Those models aren't the only things in the arsenal favoring a winter storm, possibly of significant proportions.

PSU

This final graphic here shows you 500mb height anomalies across the Northern Hemisphere on October 27th. If we look closely over Japan, and if you animate it here between frames T-10 and T-6, you can see a small but powerful trough rotating around into central Japan. Using the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the US 6-10 days later, we might expect a blast of cold air with an associated possibly-strong storm system in a November 2-7 time period, very close to (if not completely encompassing) the projected timeframe for this coastal storm in the Northeast.

Finally, just for a scope of how tricky these temperature profiles might be since it's fall, check out the 12z GFS snowfall forecast with this storm.

Instant Weather Maps

We do see some slight accumulation in New York, but it's not until the storm hits southern Canada, west of Nova Scotia, that heavy snowfall projections start to come up. This will, of course, change in coming days, and I'll update as needed. High uncertainty remains an issue in this forecast.

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